# C++ named requirements: Predicate

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The Predicate requirements describe a callable that returns a BooleanTestable value.

Predicate is typically used with algorithms that take input data (individual objects/containers) and a predicate, which is then called on input data to decide on further course of action. Some examples of predicate usage in C++ standard library are:

- std::all_of, std::any_of, std::none_of Take an array of elements and a predicate as an input. Call predicate on individual input elements, and return true if for all/any/none elements, predicate returns true.

- std::find_if Take sequence of elements, and a predicate. Return first element in the sequence, for which predicate returns value equal to true.

Description of algorithm facilities, given above, is crude and intended to explain Predicate in simple terms. For detailed info, refer to individual pages.

In other words, if an algorithm takes a Predicate `pred` and an iterator `first`, it should be able to test the object of the type pointed to by the iterator `first` using the given predicate via a construct like if (pred(*first)) { /*...*/ }.

The function object `pred` shall not apply any non-constant function through the dereferenced iterator and must accept a `const` argument, with the same behavior regardless of whether the argument is `const` or non-`const`. This function object may be a pointer to function or an object of a type with an appropriate function call operator.

### Requirements

- FunctionObject

| This section is incomplete Reason: better describe actual requirements

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3031 | C++98 | requirements on `const` values were insufficent | requirements strengthened

### See also

predicate (C++20) | specifies that a callable type is a Boolean predicate (concept)

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